When batteries are charged or discharged, they typically generate heat due to a finite internal resistance that includes ohmic, mass-transfer, and kinetic terms. Exothermic side reactions can also generate heat within the battery. This heat generation can pose a safety risk if it is large and rapid. For instance, commercial Li-ion cells generally go into thermal runaway if the internal cell temperature climbs above the decomposition temperature of the cathode (˜180 to 220° C., depending upon the chemistry and the state of charge). Often, the events that lead to a temperature rise above this critical temperature are triggered at much lower temperatures. For example, exothermic anode film decomposition can occur at ˜120° C., providing enough energy to raise the battery temperature above 180° C.
Even at lower temperatures, undesired damage can occur. For example, at milder temperatures (40 to 100° C. for Li-ion batteries), aging of the battery is usually accelerated. This is due to the fact that most detrimental side reactions are thermally activated (although not all aging mechanisms in batteries are accelerated at high temperature). It is therefore advisable to cool batteries during operation and/or at high ambient temperatures in order to enhance their cycle and/or calendar life. There are numerous cooling concepts for commercial batteries, including active air cooling, liquid cooling, the use of phase-change materials, and the use of materials with high thermal conductivity.
While battery integrity and health are significant concerns, the incorporation of batteries such as Li-ion batteries, into vehicle implicates additional concerns. Overheating of vehicular batteries during operation or storage may cause cathode materials to release oxygen gas, which reacts exothermally with the organic electrolyte. Such runaway reactions can also be caused by metallic impurities or Li dendrites that causing short-circuits between the anode and the cathode.
Therefore, being able to monitor temperature of Li-ion cells is critical in preventing catastrophic failures, as well as maintaining optimal health of batteries by reducing thermal aging. Studies show that during runaway events the temperature distribution in the cell is very non-homogeneous and it may take minutes for the thermal runaway reaction to accelerate and become irreversible. The surface temperature of the cell, however, may not show any change until it is too late to mitigate the failure. So advance knowledge of the internal cell temperature is very important, and the best way to obtain internal cell temperature is via a temperature sensor embedded inside the cell.
Incorporation of a temperature sensor into a cell is not easily accomplished. For example, the design and manufacturing process of Li-ion cells are very cost-sensitive and have been thoroughly optimized for the 18650 geometries and larger formats. In Li-ion cells of this format, a multiple-layer cathode-anode sandwich or roll is typically hermetically encased in a steel container. It is difficult and costly to re-design a cell that would include a thermistor inside with a dedicated par of contacts on the exterior casing of the cell.
What is needed therefore is a system which provides timely information regarding the internal temperature of a cell. Such a system which does not implicate significant design changes to the cell would be beneficial.